The present invention relates to a camera lens with a long back focal distance and a large aperture ratio.
Conventionally, as the aperture ratio of the lens is increased, the back focal distance tends to become shorter. When an effort is made to maintain abberations in an acceptable range in the case of a Gaussian type lens having a 1:1.4 aperture ratio and an approximately 46.degree. field angle, usually the back focal distance is about 60% of the focal length of the whole optical system of the lens. A method for improving aberrations while increasing the back focal distance is disclosed in Japanese patent publication No. 10083/1973. The publication discloses an invention capable of increasing the whole length of a lens up to 90% to 110% of the focal length of the whole optical system. However, the back focal distance of the lens only reaches 70% to 71% of the focal length of the whole optical system, so that the lens system tends to be oversized or the quantity of light in the peripheral portion of the lens tends to be reduced. Moreover, in order to prevent the increase of aberrations by making the lens small in size, it has been proposed to use glass having a high refractive index and a medium dispersion in a positive lens or a positive lens in a front group of lenses of a camera. A shortcoming of this method, however, is that correction of chromatic aberration cannot be done well.
Furthermore, in general, the back focal distance is decreased by increasing the aperture ratio of a lens. In the case of a Gauassian type lens having a 1:1.4 aperture ratio and an approximately 46.degree. field angle, as the back focal distance increases beyond 60% of the focal length of the lens, various aberrations, including coma flare, become quite conspicuous so that correction of such aberrations becomes difficult.